Ascorbic acid or vitamin C and its derivatives are commonly used as a result of their many beneficial properties. In particular, ascorbic acid stimulates the synthesis of the connective tissue and in particular of collagen, strengthens the defenses of the cutaneous tissue against external attacks, such as ultraviolet radiation and pollution, compensates for vitamin E deficiency of the skin, depigments the skin and has a role in combating free radicals. These last two properties make it an excellent candidate as cosmetic or dermatological active principle for combating ageing of the skin or for preventing ageing of the skin. Unfortunately, because of its chemical structure (of alpha-ketolactone), ascorbic acid is highly sensitive to certain environmental parameters and in particular to oxidation phenomena. There thus ensues rapid decomposition of formulated ascorbic acid in the presence of these parameters and more particularly in the presence of oxygen, light or metal ions, as a function of the temperature or under certain pH conditions (Pharm. Acta. Helv., 1969, 44, 611-667; STP Pharma, 1985, 4, 281-286).
Furthermore, it is known to use salicylic acid and its derivatives in topical compositions, in particular cosmetic or dermatological topical compositions, for example as keratolytic agent for treating acne or as antiageing agent. The documents FR-A-2 581 542 and EP-A-378 936 describe such derivatives.
However, the use of ascorbic acid or salicylic acid or their derivatives in emulsions, in particular in oil-in-water emulsions, especially when they comprise a surfactant system comprising an ester of fatty acid and of polyethylene glycol, and an additional surfactant chosen from esters of C16-C22 fatty acid and of sorbitan and esters of C16-C22 fatty acid and of glycerol, has a tendency to destabilize the emulsion, which then exhibits a phase separation of oil at the surface. The oil globules dispersed in the aqueous phase have a coarse appearance, rendering the emulsion nonhomogeneous.
In point of fact, this surfactant system described above is advantageous for the specific texture which it confers on these emulsions. The latter have a thick texture and are suitable for packaging of the cosmetic product in a pot (in contrast to fluid textures, which are packaged in a tube or in a pump-action spray; the thick texture is such that the composition does not flow instantaneously on turning the pot upside down); such a composition can be easily taken up with the fingers and spreads well and pleasantly over the skin while readily penetrating it; it confers softness and does not exhibit a tacky effect.